The present invention relates to a device for cutting plastic lenses for eyeglasses and in particular to a machine for cutting a curved lens surface and simultaneously cutting a complementary lap used to fine and polish the lens surface.
Plastic eyeglass lenses are typically produced from a molded semi-finished lens blank supplied to opticians. The semi-finished blanks are provided with the front surface of the lens already finished. Blanks are available with a variety of front surface curves known as the lens base curve. The optician is responsible for cutting or machining the inside, or concave curve, on the blank to produce a lens to the prescription of the customer.
The proper radius of the inside curve is a function of the lens base curve, the refractive index of the lens material and the customer's prescription. For a given prescription, a lens blank of a preferred base curve is selected and the inside radius is calculated based on the above known variables. The inside lens surface is cut into the blank using a machine known as a generator.
Once the concave surface is cut into the lens blank, the cut surface is fined and polished on a cylinder machine. This is typically performed by placing the concave lens surface onto a convex lap or tool of the same curvature. With a fining or polishing material located therebetween, the lens and the lap are moved relative to one another in an orbital or similar motion to fine or polish the lens surface. Once the concave surface has been polished, the lens is cut to fit the desired frame, the edge is finished, the lens is tinted and any surface coatings are applied.
Due to the expense of equipment required to cut the lens surface and finish the lens, many opticians must send their customer's orders to a central lab for production of the lenses. This necessitates a two to four day minimum wait before the customer receives his or her eyeglasses. Many larger optical outlets have a sufficient volume to justify the expense of a generator, enabling much quicker service to be provided.
In a typical on-site lab, the optician will have a supply of laps ready made to a variety of different curvatures to fine and polish the lenses. However, the laps are provided with a certain increment in radius between each lap. In order to fine and polish a lens on one of the available laps, it may be necessary to slightly alter the lens inside curvature from that calculated based on the prescription. If the increments between laps are small enough, the change is insignificant and not noticeable by the customer. To provide a sufficient supply of ready made laps to meet most of customer demands, a large quantity of laps must be purchased and stored when not in use. This can add significantly to the total cost of the laboratory equipment needed by the optician in order to provide quick service.
To remedy the inventory problem with laps, recent advances in the lens cutting art have resulted in machines that are capable of cutting a lens and subsequently cutting a lap to the same curvature. However, the currently available machines are complex and therefore available only at a high cost. The high cost makes the machines only practical for high volume labs. These machines will first cut the lens surface and then, using the same tooling, cut a complementary shaped lap. Such a machine, while overcoming the disadvantage of using stocked laps, requires two sequentially cutting operations. In addition, the generator requires complex controls and cutting mechanisms to form both a concave and convex surface.
Accordingly, it is an object of the present invention to provide an inexpensive lens generator that is capable of producing both a lens and a complementary lap.
It is a further object of the invention to provide a generator that will simultaneously cut both the lens and the lap to reduce the manufacturing process time for the lenses.
The present invention consists of a rotary cutter, such as a router, mounted to a fixture for oscillating motion of a cutting tool within a plane. The cutter is mounted to a fixture that oscillates about a rotation axis with the cutting tool extending radially outward. As a result, the cutting tool will cut a concave surface. The cutter is adjustable radially relative to the rotation axis so as to enable adjustment of the radius cut by the tool.
The lens blank is held in a fixture that is movable in an arc in a second plane transverse to the plane of oscillation of the cutter. The position of the lens blank is adjustable radially relative to the axis of rotation of the cutter to produce a finished lens having a desired thickness. With the lens blank moving in an arc in one plane and with the cutter oscillating in an arc in a transverse plane, a spherical surface is cut in the lens blank. Further adjustment capability is provided to move the axis of the cutter relative to the rotation axis of the lens to enable the production of a compound lens having a vertical radius different from the horizontal radius.
In the preferred embodiment, the cutter oscillates in a vertical direction and plane while the lens blank travels in an arc in a horizontal plane.
A second cutter is mounted to the oscillating fixture holding the lens cutter but oriented so that its cutting tool extends radially inward toward the oscillation axis. As a result, a convex curve is cut as opposed to the concave curve of the lens cutter. Another mounting fixture is provided to mount a lap oriented in the opposite direction relative to the lens blank. The lap mounting fixture is movable together with the lens blank mounting fixture. As the lap cutter oscillates in one plane and the lap travels in a transverse plane, a convex surface is cut into the lap of the same radius as the concave surface cut in the lens blank. The lap is then used to fine and polish the lens. By custom manufacturing of the lap to fit a particular lens, the lens and lap are made to the exact radius needed for the prescription. There is no need to adjust the lens radius to fit the available laps used to fine and polish the lens.
The lens generator of the present invention is a relatively simple mechanical device that enables the machine to be manufactured and sold at a significantly lower cost than currently available generators that produce both a lens and lap. As a result, an optician with a relatively low volume is able to justify the cost of a generator, thereby enabling him or her to provide on-site service in a short period of time.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.